Nuclear Chemistry

1
Nuclear Chemistry

• Chapter 10 Prentice Hall Physical Science

2
Review

• All the chemistry weve discussed so far has
  involved electrons.
• Questions
• If element X has a molar mass of 3 g/mol and
  element Y has a molar mass of 5 g/mol, what must
  be the molar mass of X2Y?
• If you tossed 128 coins in the air, about how
  many would you expect to land heads-up?
• What do the mass number and atomic number
  represent?
• Which subatomic particles are found in the
  nucleus?

3
Radioactivity

• Antoine Henri Becquerel (1896) experimented with
  uranium salts and discovered radioactivity
• Radioactivity (or nuclear decay) an unstable
  nucleus emits charged particles and energy
• Radioisotope radioactive isotope - any atom that
  has an unstable nucleus. Examples
• Uranium-238 (used in Becquerels experiment)
• Carbon-14 (used often in radioactive dating)

4
Isotope symbology

• Isotopes are named using the element name
  followed by the mass number (see examples, slide
  3)
• The symbol for isotopes includes the element
  symbol, the mass number and the atomic number as
  follows

Mass on top
Atomic on bottom
5
3 Types of Nuclear Radiation

• Nuclear radiation charged particles and energy
  that are emitted from the nuclei of radioisotopes

Radiation Type Symbol Charge Mass (amu) Common Source
Alpha particle a, 2 4 Radium-226
Beta particle b, 1- Carbon-14
Gamma ray g 0 0 Cobalt-60
6
Alpha Decay

• Alpha particle, a
• 2 protons and 2 neutrons
• Positively charged
• Same as He nucleus
• Least penetrating type of nuclear radiation
• Travel only centimeters in air
• Can be stopped by a sheet of paper or clothing

7
Beta Decay

• Beta particle, b
• 1 electron
• Negatively charged
• Produced by a neutron that decomposes into a
  proton and an electron
• More penetrating than a particles
• Pass through paper
• Stopped by a thin sheet of metal

8
Gamma Decay

• Gamma ray, g
• Penetrating ray of energy
• Like X-rays and light, only very short wavelength
• Most penetrating form of the three types
  discussed
• Often accompanies alpha or beta decay
• Several centimeters of lead or several meters of
  concrete required to stop it

9
Writing and Balancing Nuclear Reactions

• Similar to chemical equations, but isotope
  symbols are used.
• In a balanced nuclear equation
• Mass on the left sum of mass s on the right
• Atomic on the left sum of atomic s on the
  right
• You will need to use your PERIODIC TABLES!

Reactants ? Products
10
Example Math Skills p. 295

• Write a balanced nuclear equation for the alpha
  decay of polonium-210.
• Step 1 Define reactants and products. Use
  letters to represent the unknown values.
• Step 2 Write and solve equations to find unknown
  atomic and mass s.
• Step 3 Look up the element symbol on the
  periodic table using the atomic .
• Step 4 Write the balanced nuclear equation and
  double-check your solution.

Atomic 82 Pb (Lead)
11
Effects of Nuclear Radiation

• Background radiation naturally occurring in the
  environment
• Sources
• Radioisotopes in air, water, rocks living
  things
• Cosmic radiation
• Generally at safe levels
• Nuclear radiation can ionize atoms. At levels
  significantly above background, this can damage
  DNA and proteins
• Which type of nuclear radiation is the least
  harmful? Which the most?

12
Detecting Nuclear Radiation

• Geiger counters
• Use gas-filled tubes to measure ionizing
  radiation
• Gas produces an electric current when exposed to
  ionizing radiation
• Film badges
• Photographic film wrapped in paper
• Film is exposed with exposure to radiation like
  photographic film is exposed with exposure to
  visible light

13
Rate of Nuclear Decay

• Nuclear decay rate describes how fast nuclear
  changes take place
• Unlike chemical reactions, nuclear decay rate
  does NOT vary with external conditions it is
  constant for a given radioisotope
• Half-life the time required for half of a
  radioisotope sample to decay

14
Rates of Nuclear Decay (contd)
15
Rates of Nuclear Decay (contd)

• Different radioisotopes have different half-lives
• To determine how many half-lives have elapsed for
  a sample, divide the total time of decay by the
  half-life
• Known decay rates are used in radioactive dating

Radioisotope Half-life
Radon-222 3.82 days
Iodine-131 8.07 days
Carbon-14 5730 years
Thorium-230 75,200 years
Uranium-238 4.47x109 years
16
Radiocarbon dating

• Carbon-14 exists naturally in the atmosphere at a
  fairly constant ratio to C-12

CO2 absorbed while living (including some C-14)
Age of fossil determines by comparing C-14/C-12
ratio in fossil to atmospheric ratio
As C-14 decays, its replaced by C-14 absorbed
from atmosphere
Tree dies no more CO2 absorbed to replace
decaying C-14
17
Radiocarbon dating (contd)

• Used for objects less that 50,000 years old
• For older objects, must use different isotope
  with longer half-life
• What isotopes would work well to date a rock
  formation that is thought to be close to a
  trillion years old?

18
Artificial Transmutation

• Transmutation conversion of atoms of one element
  into atoms of another
• Alchemists have attempted this for hundreds of
  years (but not through nuclear chemistry)
• First artificial transmutation Ernest Rutherford
  (1919) turned nitrogen into oxygen-17

19
Artificial Transmutation (contd)

• Transmutation achieved by bombarding atomic
  nuclei with high-energy particles
• Protons, neutrons or alpha particles
• Example Ernest Rutherfords transmutation used
  which particle?
• Transuranium elements
• Many produced by artificial transmutation of a
  lighter element
• All are radioactive

20
Nuclear Forces
Strong nuclear forces
Electric forces

• The strong nuclear force attracts protons and
  neutrons.
• Stronger than electric forces over short
  distances
• Decreases with distance (like gravity)
• Electric repulsions push protons apart.
• When a nucleus is large enough, the electric
  forces can overcome the strong nuclear forces.
• Nuclei are unstable at this point.
• Any atom with 83 or more protons is unstable
  and, therefore, radioactive.

Proton from a small nucleus
Small nucleus
Proton from a large nucleus
Large nucleus
21
Fission

• Fission splitting of nucleus into two smaller
  parts
• Lise Meitner, Fritz Strassman and Otto Hahns
  experiments (1939) first demonstrated nuclear
  fission.
• A small amount of the original mass is converted
  into a lot of energy

22
Fission (contd)

• About how much energy was released from 6.2 kg of
  Plutonium-239 in the second atomic bomb
  explosion? (Note Only about 1 kg underwent
  fission the rest was scattered.)

This quantity 2.5 x 1010 kWh, or enough energy
to power my house for over 3.6 million years!
23
Fission and Chain Reactions

• Fission can result in a chain reaction.
• Neutrons released from the first reaction can
  trigger another reaction, and so on similar to
  a rumor spreading.

Neutron
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Chain Reactions (contd)

• For a chain reaction to happen, each split
  nucleus must produce at least one neutron with
  enough energy to split another nucleus
• This only happens when a specific mass of
  fissionable material is available called the
  critical mass.
• Controlled chain reactions are used to generate
  electricity in nuclear power plants.
• Uncontrolled chain reactions are used in nuclear
  weapons

25
Nuclear Fusion

• Fusion nuclei of two atoms combine
• The sun and other stars are powered by fusion of
  H into He
• Requires extremely HIGH temperatures
• What state is matter in at such high
  temperatures? PLASMA